Tuesday, December 23, 2008

Maybe A Cold Earth in the Eoarchaean Isn't A Problem

For two decades or more, biologists have had a problem with astronomical and geological accounts of the early sun's evolution. The problem is that our models show that the sun was probably too faint to maintain liquid water on the Earth's surface until sometime in the Paleoarchaean, and solid ice makes a poor chemical reactor for early evolution. In 2007 Daupha et al showed evidence of mineral deposition 3.8 Ga under conditions of high-pressure carbon dioxide, which of course means a probable greenhouse effect at that time on Earth.

Because we've always assumed early Earth was a hot Earth, the conventional wisdom of the evolution of life on Earth has held that the last universal common ancestor (LUCA) of all living things was a thermophile, much like the biochemically weird things living in deep sea vents or those prismatic mats at Yellowstone, which is why an early cold Earth doesn't seem to fit. But a snowball or at least near-snowball ancient Earth may not be a problem after all. A recent publication by Boussau et al shows that LUCA was a mesophile, best adapated to life at temperatures below 10 degrees C.

This is also consistent with the idea of an RNA world, since RNA is unstable at low and high pH, and even less stable at high temperatures than DNA. Interestingly, the LUCA of eubacteria and archaebacteria does look like a thermophile, consistent with temperature estimates of later periods in Earth's history when the ocean was thought to be on average 80 degrees C (about 3.5 Ga). The resolution of ancestors at different time depths is permitted by the use of two methods, one based on GC content of the genome and one based on amino acid substitution.

This is biological and geological convergence at its finest; Lyell and Darwin (and Woehler and Mendel) would be proud. We now have a consistent story for the evolution of the sun, the atmosphere of early Earth and its effect on surface temperature, the temperature adaptation of the earliest cells and the switch from RNA to DNA. The next step is to explore the period 3.8 to 3.5 Ga, to look for evidence of skyrocketing ocean temperatures; not inconceivable given a thick CO2 atmosphere and a sun that was still booting up. If our picture of Earth's surface temperature in that period is accurate, we can also start to ask whether rising temperatures were the selection pressure switching the early biosphere from RNA to DNA.

Words to Live By

"...there is good and bad speculation, and this is not an unparalleled activity in science...Those scientists who have no taste for this sort of speculative enterprise will just have to stay in the trenches and do without it, while the rest of us risk embarrassing mistakes and have a lot of fun." - Dan Dennett